1. Field of the Art
The present invention relates to an apparatus for machining surfaces of magnetic data recording discs, and more particularly to an apparatus for forming a textured zone in a predetermined surface area of magnetic data recording discs.
2. Prior Art
On a magnetic disc drive, information is recorded on or read out from a magnetic data recording disc (hereinafter referred to simply as "disc"for brevity) by means of a magnetic head which is positioned face to face and in a predetermined small gap relation with a surface area of the disc which is put in rotation. When the disc is at rest, the magnetic head is located in a predetermined receded position in contact with a disc surface, and, upon starting rotation of the disc, the magnetic head is initially allowed to slide on the disc surface and then floated up by air streams which occur between the magnetic head and the disc surface as the rotational speed of the disc is increased to a rated steady level. The uplifted magnetic head is displaced to a data recording zone for a data write-in or read-out operation. Upon stopping the rotation of the disc, the magnetic head is moved back to the predetermined position again, and, as the disc floating force weakens with deceleration of the rotational speed of the disc, it is lowered into sliding contact with the disc surface and allowed to rest standstill in that position as soon as the disc comes to a stop.
This sort of magnetic head control is generally referred to as a "contact start-stop (CSS)" type, and a CSS zone where a magnetic head is retained at rest is provided on the disc surface in addition to the data recording zone to be accessed for data write-in and read-out operations. In this regard, the CSS zone of this nature is normally provided in the form of an annular band of a predetermined width in the proximity of inner marginal edges of a disc, and, for the purpose of floating up the magnetic head smoothly with less friction, its surface is roughened to some extent by imparting minute surface irregularities generally referred to as "texture".
For texturing the CSS zone, it is necessary to form minute surface irregularities of uniform shapes of an extremely high accuracy level. Especially, in order to meet recent demands for higher magnetic recording densities, it is a paramount requisite to control the uplifting of the magnetic head to an infinitesimally small height above the disc surface and, for this purpose, it is necessary to improve the accuracy of texturing operations all the more by making surface irregularities on a textured surface as fine and uniform as possible.
As for the texturing technology, there are a number of surface texturing methods available, including texturing methods by tape grinding and chemical etching and newly developed texturing methods by laser beam bombarding. Of these texturing methods, the laser texturing method is most advantageous for use in texturing limited surface areas like CSS zones on magnetic data recording discs.
Laser texturing apparatus for magnetic data recording discs is normally constituted by a laser energy source, an optical system at least including a laser beam collimating means, a laser beam bombarding means including an objective lens, and a spindle means including a spindle shaft with a disc holder member or members at its distal end and a rotational drive means such as electric motor or the like. While shooting off laser pulses repeatedly from the laser energy source of the laser beam bombarding means, the beam spot of the laser pulses is narrowed to a predetermined spot diameter through the objective lens and bombarded on a disc surface. The bombarding of laser pulses result in formation of crater-shaped surface irregularities which are generally called "bumps".
The above-mentioned CSS area needs to be formed only in a limited width on a disc surface, so that, during a texturing operation, a laser beam bombarding position on a rotating disc is gradually displaced over a predetermined range in a radial direction of the disc, forming bumps in a spiral pattern on the disc. Namely, while a disc is being retained in position on and put in rotation by the spindle means, the laser beam bombarding means is moved in a radial direction of the disc. Accordingly, for this radial shift of the laser beam bombarding position, at least the objective lens of the laser beam bombarding means is moved in a radial direction in a plane parallel with the disc surface. By radially shifting the laser beam bombarding position relative to the rotating disc in this manner, an annular textured zone of a predetermined width is formed on the surface of the disc. More specifically, a surface area on the inner peripheral side of a disc is textured over a predetermined limited width for the CSS zone, by radially moving the objective lens of the laser beam bombarding means relative to the rotating disc on the spindle means, while controlling the laser beam bombarding means to start shooting off laser pulses at predetermined time intervals from a bombarding start position as soon as the objective lens comes into a radial position confronting an inner marginal edge of a CSS zone area to be formed on the disc, through to a bombarding end position confronting an outer marginal edge of the CSS zone.
In this connection, the accuracy of textured surfaces has extremely great influences on the control of the magnetic head uplifting action. In particular, in order to cope with the recent trend toward high density magnetic data recording discs which are drastically enhanced in magnetic recording density, it is essential to control the uplifting of the magnetic head to an infinitesimally small height. For this purpose, bumps have to be formed with high precision especially in diameter and pitches in rotational and radial directions. More specifically, the CSS zones on magnetic data recording discs are generally textured with bumps of several .mu.m in diameter in a pitch of several .mu.m in the rotational direction and in a pitch of several .mu.m to several tens .mu.m in the radial direction. There are, however, still increasing demands for a texturing machine of higher precision in terms of bump diameter and bump pitches in the rotational and radial directions.
Texturing machines for magnetic data recording discs necessarily involve mechanical drives for rotating a disc which is set on the spindle means, and for moving at least part of the laser beam bombarding means in a direction parallel with the machining disc surface. In this regard, although dimensions of discs are strictly controlled in disc fabrication processes, individual discs normally have a dimensional tolerance of about .+-.25 .mu.m in inner diameter, that is to say, there is a difference of about 50 .mu.m in inner diameter between discs with the maximum and minimum tolerable inner diameters. Therefore, when setting these discs one after another on the spindle means by a clamp member or members of a disc holder means which is arranged to hold inner peripheral edges of each disc, there may arise difficulties in setting discs of the minimum tolerable inner diameter in position on the holder member unless the outside diameter of the disc holder means is reduced further to an appreciably smaller size as compared with the minimum tolerable inner diameter.
As a consequence, more or less there inevitably occurs a positional deviation to a disc which is set on the spindle means by the disc holder means, more specifically, a deviation of the rotational center of a disc from the rotational axis of the spindle shaft, due to the tolerance range in inner diameter and varying differences in inner diameter of individual discs from the outer diameter of the disc holder member. Such a positional deviation of a disc is reflected by a deviation of the bombarding start position of the laser beam bombarding means relative to the disc which is set on the spindle means, and this relative positional deviation remains uncorrected even at the bombarding end position. A certain degree of positional deviation of a textured CSS zone toward the inner peripheral side of a disc would not give rise to problems in particular because inner peripheral edges of the disc will be chucked on a spindle of a disc drive unit over a width of several millimeters from its inner marginal edges. On the other hand, a radially outward deviation of the CSS zone toward a data recording zone of the disc could result in serious problems, so that a CSS zone should be clearly bordered from a data recording zone at a correct position under strict control. However, radial deviations of individual magnetic data recording discs from the rotational axis of the spindle would make it difficult to form on each disc a CSS zone which is clearly bordered from a data recording zone invariably at a predetermined radial position. Further, the positional deviations of this nature could lead to unexpected irregular variations in pitch of bumps in the radial direction.